The present invention relates to a process for making polycrystalline cubic boron nitride (CBN). The manufacture of CBN by a high pressure/high temperature technique is known in the art and is typified by the process described in U.S. Pat. No. 2,947,617 of Wentorf, a basic monocrystalline CBN case. U.S. Pat. No. 4,188,194 describes a process for making sintered polycrystalline CBN compacts which utilizes pyrolytic hexagonal boron nitride (PBN) in the absence of any catalyst. A compact is a mass of abrasive particles bonded together in a self-bonded relationship (see U.S. Pat. Nos. 3,852,078 and 3,876,751); by means of a bonding medium (see U.S. Pat. Nos. 3,136,615, 3,233,988, 3,743,489, 3,767,371, and 3,918,931); or by means of combinations thereof. U.S. Pat. No. 3,918,219 teaches the catalytic conversion of hexagonal boron nitride (HBN) to CBN in contact with a carbide mass to form a composite body. A CBN compact is comprised of a plurality of CBN crystals suitably bonded together to form a large, integral, tough, coherent, high-strength mass. Compacts may be used in applications such as, for example, machining, dressing, and drilling (see U.S. Pat. Nos. 3,136,615 and 3,233,988).
Boron-rich polycrystalline CBN as used in the subject invention can be prepared by high pressure/high temperature processing of mixtures of HBN powder and either elemental boron or various boron containing compounds (e.g. AlB.sub.12), as described in British Pat. No. 1,513,990; or using vacuum-fired HBN powder to produce excess surface elemental boron which then is converted to CBN as described in U.S. Pat. No. 4,289,503. In these processes, the large chunks of polycrystalline CBN which are produced typically are milled to a size suitable for use in various grinding or other applications. In the milling process, a portion of the material normally is crushed too fine or has an undesirable shape for use in grinding applications and, thus, some use for such unwanted material is desirable. Also, current commercial polycrystalline CBN compacts could be improved by increased edge strength and decreased brittleness (lack of impact strength). An improved polycrystalline CBN product is desired.